The present invention relates to an electrode material for a battery and electric double layer capacitor, a method of producing the same, a battery, and an electric double layer capacitor, particularly to an electrode material which is most suitable for forming an electric double layer capacitor having a large capacitance and large current discharge capability, and a method of producing the same.
As materials having the capacity to adsorb various substances or ions, for example, powdered activated carbon, granular activated carbon and fibrous activated carbon have been known and are widely used as electrode materials for various batteries, various adsorbents used in water purifiers, deodorization apparatuses, decoloration apparatuses or the like and catalyst carrier. These activated carbons are prepared by activating a carbon material mainly made from coconut shell, coal, lumber or bamboo as a raw material in the presence of steam or in the presence of zinc chloride or potassium hydroxide, and include numerous fine pores therein.
These fine pores are generally referred to as submicropores when the pore diameter is smaller than 0.7 nm, micropores when the pore diameter is within a range from 0.7 to 2.0 nm, mesopores when the pore diameter is within a range from 2.0 to 50 nm, and macropores when the pore diameter is 50 nm or larger.
According to a conventional method of preparing the activated carbon, although activated carbon including micropores having a pore diameter within a range from 0.7 to 2.0 nm and submicropores having a pore diameter smaller than 0.7 nm developed therein are made, formation of mesopores having a diameter within a range from 2.0 to 50 nm is insufficient and the volume of mesopores account for less than 10% of the total volume of pores. Such an activated carbon has a large specific surface area and excellent capability to adsorb molecules having a size smaller than 2.0 nm, but is not capable of efficiently adsorbing and desorbing an organic compound and an inorganic compound, which are used as an electrolyte of the electric double layer capacitor, and aggregates of larger size whichare formed through solvation of these substances.
In view of the adsorption and desorption properties appropriate for the molecule size of the adsorbed material, it is desirable to prepare an activated carbon that has pores of only a specific size. However, no adsorbent having a pore size distribution that is specific in a particular range of pore sizes has been obtained in the mesoscopic pore range.
Although such an activated carbon that supports transition metals or transition metal compounds and has a catalytic action to decompose a material adsorbed onto the activated carbon and electrode materials having a large capacitance have been reported no activated carbon-supported transition metal having a pore size distribution that is specific in a particular range of pore sizes has been obtained in the mesoscopic pore range, as described above. Also, because the activated carbon is processed to support the transit ion metal""s or transition metal compounds by adsorption of the transition metals or transition metal compounds after the activated carbon has been prepared, in case the activated carbon is immersed in an electrolytic solution for use as an electrode material, for example, there is such a problem that the supported metal elutes. Therefore, an electrode material that has satisfactory electrical characteristics and stable charge and discharge characteristics has not been made available.
In recent years, demands for an electric double layer capacitor that utilizes activated carbon as the electrode material have been increasing as the backup power source, auxiliary power source and other uses, and are attracting much attention with the development of the electronics industry.
More recently, it has been called for to reduce the size of memory backup power sources further, and develop a secondary battery that can be used as an auxiliary power source having a large capacity and capability to supply a large current instantaneously such as the onboard power source for vehicles.
Activated carbon has a large specific surface area and high chemical stability, and therefore, electrode materials that consist mainly of activated carbon are used as both positive and negative electrodes, for the polarizing electrodes of an electric double layer capacitor.
While the capacitance of the electric double layer capacitor is dependent on the specific surface area, packing density and internal resistance of the electrode material and other factors, particularly important is the relationship between the size of the electrolyte ions included in the electrolytic solution that forms the electric double layer and the size of the pores formed in the electrode material.
In those that are referred to as the electric double layer capacitors of an organic solvent base that utilize an ammonium ion, phosphonium ion or the like, among the electric double layer capacitors, it is said that pores having a pore diameter of 2 nm or larger in the electrode material contribute to the capacitance. Also, in a water-based electric double layer capacitor that uses sulfuric acid as the electrolyte, it is believed that pores having a pore diameter of 2 nm or larger contribute to the performance such as capacitance and current density. Thus, it is expected that excellent materials for electrodes of an electric double layer capacitor or battery will be made from an activated carbon that includes pores having a pore diameter within a range of Xxc2x1xcex1 nm (3.0xe2x89xa6X less than 10, xcex1=1.0; range of pore size distribution) of which volume accounts for 15% or more of the total volume of mesopores having a pore diameter within a range from 2.0 to 50 nm.
When using a metal supporting activated carbon made by dispersing a large amount of a transition metal or a transition metal compound in an activated carbon as the electrode material, electrolyte that has migrated into the electric double layer is taken into the transition metal or transition metal compound that is present in the activated carbon, and therefore, a greater amount of energy can be stored in comparison to a case in which the electric double layer is used individually. Thus, an activated carbon that includes pores having a pore diameter within a range of Xxc2x1xcex1 nm (3.0xe2x89xa6X less than 10, xcex1=1.0; range of pore size distribution) of which volume accounts for 15% or more of the total volume of mesopores having a pore diameter within a range from 2.0 to 50 nm, and contains 0.01 to 50% by weight of a transition metal or a transition metal compound will make excellent electrodes for an electric double layer capacitor or a battery.
The thickness of the electric double layer formed from ions that are adsorbed onto the inner surface of pores in an electrode material is believed to be about 1 nm. Since the pore diameter must be 2 nm or larger in order to form the electric double layer evenly on the inner surface of the pores, a conventional activated carbon of which pores consist mainly of micropores having a pore diameter of 2 nm or less is not suited for use as an electrode material for large capacitance and large current discharge. Moreover, in a conventional activated carbon of which pores consist mainly of micropores having a pore diameter of 2 nm or less, the speed of migration,of the electrolyte ions generated through salvation becomes slower which is believed to make such an activated carbon unsuited for use as an electrode material for large capacitance and large current discharge.
Accordingly, an activated carbon, which is made by supporting a transition metal or transition metal compound on the conventional activated carbon of which pores consist mainly of micropores having a pore diameter of 2 nm or less, does not have such pores for the electric double layer to be formed evenly over the inner surface thereof, and the speed of the electrolyte ions to migrate in the pores becomes slower, and therefore, the activated carbon of the prior art is not capable of storing a large amount of energy in the transition metal or transition metal compound and efficiently utilizing the energy. These problems are believed to make the conventional activated carbon that supports the transition metal or transition metal compound unsuitable as the electrode material for large capacitance and large current discharge.
Meanwhile, metal-halogen batteries, for example, a zinc-bromine battery, have been vigorously developed for the reason of such excellent features as the active materials used to make both electrodes are available in abundance at a low cost, theoretical energy density is high, output of the battery can be easily controlled due to the liquid circulating operation, and maintenance work is made easy because the battery with aqueous solution which operates at a low temperature. An activated carbon has been examined as a promising candidate for the surface treating material of the positive electrode of these batteries. However, several problems must be solved before making practical use of such a battery. Among others, it is an important technical task to form pores, consisting mainly of mesopores having a pore diameter of 2 nm or larger, in the activated carbon used as the electrode material for the positive electrode, since the energy efficiency of a battery is directly affected by how fast and how efficiently the reducing reaction of halogen is carried out in the positive electrode during discharge.
Various attempts have been made to prepare adsorbents that make it possible to increase the pore diameter of the activated carbon and adsorb substances of relatively large molecule diameter. For example, there is a method of increasing the pore diameter by repeating the activation treatment many times. With this manufacturing method, however, the volume of pores in the mesoscopic range is a small proportion of the total volume of pores and the repetition of the activation treatment decreases the yield of production, and is therefore undesirable.
Unexamined Patent Publication (Kokai) No. Hei 5-302216 discloses a method of reforming carbonaceous fibers by treating the carbonaceous fibers having a specific surface area within a range from 0.1 to 1200 m2/g with an oxidizing agent to hydrophilize the fibers, supporting an alkaline earth metal on the fibers and subjecting them to an activation treatment. With this method, although pores having diameters within a range from 1.5 to 15 nm are formed, the volume of pores having a pore diameter within a range of Xxc2x1xcex1 nm (3.0xe2x89xa6X less than 10, xcex1=1.0; range of pore size distribution) does not account for 15% or more of the total volume of mesopores having a pore diameter within a range from 2.0 to 50 nm, and the microscopic structure of the carbon material is not strictly controlled. Such a carbon material has such drawbacks as a broad distribution of a pore diameter, while a significant proportion of the pores are those having diameters not effective for adsorption which leads to lower adsorption efficiency, thus decreasing the fiber density and the strength.
Unexamined Patent Publication (Kokai) No. Hei 5-000811 discloses an activated carbon material which is made from protein, sludge or waste including protein, or activated carbon in the form of polyacrylonitrile fibers. As such, the material contains a large amount of impurities. Also because the material is simply carbonized, or an activation treatment is carried out only by means of steam, carbon dioxide gas, oxygen, etc., the pore diameter is not strictly controlled. Further, because of the low impurity of the carbon, the material has a low electrical conductivity and is not suited for use as an electrode material.
Unexamined Patent Publication (Kokai) No. Hei 5-294607 discloses a method of preparing a metal-containing activated carbon by subjecting a mixture of pitch having a low softening point and a metallic compound to a carbonization or activation treatment. According to this method, there could not be obtained a material that includes pores having a pore diameter within a range of Xxc2x1xcex1 nm (3.0xe2x89xa6x less than 10, xcex1=1.0; range of pore size distribution) of which volume accounts for 15% or more of the total volume of mesopores having a pore diameter within a range from 2.0 to 50 nm. This method is also not economically feasible due to a high material cost because a rare earth compound such as a ytterbium compound and/or yttrium compound are used.
An energy storing element disclosed in Unexamined Patent Publication (Kokai) No. Hei 4-294515 and a super capacitor disclosed in Unexamined Patent Publication (Kokai) No. Hei 6-503924 are both produced by bonding a transition metal or a transition metal compound to a porous carbon material such as activated carbon, but are not an electrode material that includes pores having a pore diameter within a range of Xxc2x1xcex1 nm (3.0xe2x89xa6X less than 10, xcex1=1.0; range of pore size distribution) of which volume accounts for 15% or more of the total volume of mesopores having a pore diameter within a range from 2.0 to 50 nm, and are not suited for electrode material for large capacitance and large current discharge. Moreover, since a significant amount of the transition metal or transition metal compound is bonded to the porous carbonaceous material such as activated carbon, by the method described in the publication mentioned above, when the porous carbonaceous material is brought into contact with a high concentration solution of the transition metal or transition metal compound, the transition metal or transition metal compound cannot be dispersed sufficiently, resulting in the coagulation of the transition metal or transition metal compound. This leads to such problems as micropores of the porous carbonaceous material are clogged, or the transition metal or transition metal compound that is merely adsorbed physically onto the surfaces of the pores in the porous carbonaceous material dissolves into the electrolyte, and consequently these materials do not have sufficient performance when used as the electrode material for the electric double layer capacitor.
As described above, according to the conventional methods of forming pores in the mesoscopic range, there have never been obtained an electrode material that includes pores having a pore diameter within a range of Xxc2x1xcex1 nm (3.0xe2x89xa6X less than 10, xcex1=1.0; range of pore size distribution) of which volume accounts for 15% or more of the total volume of mesopores having a pore diameter within a range from 2.0 to 50 nm and strict control of the a pore diameter has not been achieved. Accordingly, there has never been obtained an electrode material that includes pores having a pore diameter within a range of Xxc2x1xcex1 nm (3.0xe2x89xa6X less than 10, xcex1=1.0; range of pore size distribution) of which volume accounts for 15% or more of the total volume of mesopores having a pore diameter within a range from 2.0 to 50 nm, on which the transition metal or transition metal compound is supported. The present invention has been made to solve the problems described above, and an object thereof is to provide an electrode material that includes pores having a pore diameter within a range of Xxc2x1xcex1 nm (3.0xe2x89xa6X less than 10, xcex1=1.0; range of pore size distribution) of which volume accounts for 15% or more of the total volume of mesopores having a pore diameter within a range from 2.0 to 50 nm, while having pore size distribution with the maximum value in the range, preferably having a diffraction peak originated in a graphite crystal in X-ray diffraction and contains 0.01 to 50% by weight of the transition metal or transition metal compound. Other objects of the present invention are to provide a method of producing the electrode material described above, and a battery or electric double layer capacitor that employs the electrode material.
The present inventors have intensively studied to solve the problems described above and found it possible to obtain an electrode material that includes pores having a pore diameter within a range of Xxc2x1xcex1 nm (3.0xe2x89xa6X less than 10, xcex1=1.0; range of pore size distribution) of which volume accounts for 15% or more of the total volume of mesopores having a pore diameter within a range from 2.0 to 50 nm by adding at least one transition metal or at least one transition metal compound to a carbon material and/or a carbon material precursor, followed by carbonization in a non-oxidizing atmosphere or activation in a slightly oxidizing atmosphere at a temperature higher than 600xc2x0 C., thus completing the present invention. Also the present inventors have found it possible to provide an electrode material that has a maximum value of pore size distribution in the range described above and has a diffraction peak originated in a graphite crystal in X-ray diffraction, while containing 0.01 to. 50% by weight of the transition metal or transition metal compound, thus completing the present invention.
That is, the present invention provides an electrode material that includes pores having a pore diameter within a range of Xxc2x1xcex1 nm (3.0xe2x89xa6X less than 10, xcex1=1.0; range of pore size distribution) of which volume accounts for 15% or more of the total volume of mesopores having a pore diameter within a range from 2.0 to 50 nm. The present invention also provides an electrode material that has a maximum value of pore size distribution in the range described above and has a diffraction peak originated in a graphite crystal in X-ray diffraction. The present invention further provides an electrode material that contains 0.01 to 50% by weight of the transition metal or transition metal compound. The present invention still further provides a method of producing the electrode materials, an electric double layer capacitor that has large capacitance and is capable of discharging large current and a battery of large capacity.
While the mechanism of forming the mesopores in the electrode material of the present invention has not yet been elucidated, it is presumed that the carbon skeleton surrounding the transition metal is once destroyed by the catalyst action of the transition metal during heat treatment, and the rate of reaction between the activation gas and the carbon increases significantly leading to a condition similar to erosion by the transition metal, so that the formation, expansion and congregation of the pores proceed and result in the formation of mesopores. Size of the mesopores thus formed seems to be related to the size of the transition metal atom. Partial formation of graphite crystal is also considered to occur due to the rearrangement of.:the atoms during formation of the mesopores.